PROJECT SUMMARY Evidence-based psychotherapies for posttraumatic stress disorder (PTSD) and depression consistently produce strong, clinically meaningful effects for many individuals. However, these interventions also have significant dropout rates, a large minority of individuals continue to have debilitating symptoms, and even those who respond may be vulnerable to relapse upon future stressors. More efficient and mechanistically precise interventions are needed. Consistent with the cross-cutting theme of studying the role of the environment in the NIMH Strategic Plan, the etiologic role of exposure to destabilizing, stressful life events is common to both PTSD and depression. Not only do they share common distress-related triggers, symptoms, and maintaining processes, but they also commonly co-occur (upwards of 60%). Current PTSD and depression treatments typically focus on their respective disorders rather than on common processes that maintain psychopathology; and, importantly, they do not explicitly target positive adaptive processes associated with resilience. Decades of experimental studies, prospective studies, and psychotherapy trials have identified interconnected maladaptive and adaptive processes associated with persistent psychopathology after stressful, destabilizing events. These maladaptive processes include: 1) unproductive event processing; 2) avoidance; and 3) reward sensitivity and processing deficits. These processes prolong negative mood, interfere with adaptive coping and processing of emotional material, and increase sensitivity to future stressful life events. PATH (Positive Processes and Transition to Health) directly targets these maladaptive processes while also teaching parallel adaptive skills (constructive processing, approach, and positive emotion processing and reward seeking). Six, 90-min sessions target individuals who have experienced a destabilizing life event and have persistent stressor-related symptoms. PATH utilizes life event processing (revisiting, meaning making), focusing repeatedly on an identified destabilizing life event, positive life events, and future events as a framework to identify maladaptive processes and teach constructive processing skills. In a small open trial (R61, N = 45), we will examine whether PATH engages the key targets of unproductive processing, avoidance, and reward deficits. Next, we will conduct a randomized trial of PATH (R33, N = 135), comparing PATH to a Progressive Muscle Relaxation, seeking to replicate changes in the targets in a larger sample and examine PATH's impact on stressor-related psychopathology (PTSD, depression). PATH, a brief and focused treatment that targets key psychological processes common to PTSD and MDD, has the potential to reduce dropout, improve treatment engagement and outcomes, identify potential treatment mechanisms, and ultimately reduce the costly human and economic burden of stressor-related psychopathology.